Pilgrim Station was taken offline after a “minor leak” at one of the plant’s four safety relief valves was discovered, according to a statement from Entergy Nuclear, a subsidiary of the Louisiana company that owns and operates the facility. The valves are designed to automatically reduce steam pressure in the plant’s reactor vessel, said Entergy spokesman Rob Williams.

He said the plant — it employs 650 people and is capable of generating about 688 megawatts of power — will remain out of service until investigators have analyzed the problem and made repairs. According to a company press release, “the plant is in a safe condition.”

“The focus remains on safety,” he said. “They assemble the maintenance crew — they’re trained on this type of thing — and they go in and make the fix.”

The 40-year-old plant also went offline on Jan. 10, after the station’s recirculation pumps, which are used to adjust power levels in the facility, stopped operating. The plant was reconnected to the electric transmission grid six days later. Before that shutdown, the plant had operated continuously for 230 days.

“Normally,” Williams said, “these plants are designed to run day and night.”

The shutdowns come eight months after the Nuclear Regulatory Commission voted to renew Pilgrim’s operating license for another 20 years, despite concerns among neighbors about the station’s age and Entergy’s ability to ensure the safety of surrounding populations. The plant is about 35 miles from Boston.

The breaks in operation also follow a monthlong lockout of Pilgrim workers by the company during a contract dispute last summer involving wage increases and benefits. Entergy brought in temporary workers to keep the plant running.